In this chapter, we explain the main technologies and concepts related to communications networks in railways. More precisely, “wired” communications systems, in opposition to “mobile” or “wireless,” is addressed in Chap. 5 . Moreover, in this chapter, we do not cover onboard train networks, which you’ll find in Chap. 6 . Consequently, in this chapter, we address the main technologies and systems for the wayside networks, such as synchronous digital hierarchy (SDH) (which is today seen as legacy, but still very much used) and especially, Multiprotocol Label Switching (MPLS).